Re: [PATCH v2 5/9] target/i386/kvm: Use AMD PMU MSR paths for Hygon

On Mon, Aug 10, 2026 at 04:29:52PM +0800, Tina Zhang wrote:
> Date: Mon, 10 Aug 2026 16:29:52 +0800
> From: Tina Zhang <[email protected]>
> Subject: [PATCH v2 5/9] target/i386/kvm: Use AMD PMU MSR paths for Hygon
> X-Mailer: git-send-email 2.43.7
> 
> On SVM, KVM uses the AMD PMU implementation and AMD PMU CPUID/MSR
> layout.  Hygon guests that enable PMU and request AMD PMU CPUID
> features such as perfctr-core need QEMU's KVM PMU setup and MSR state
> paths to use that layout too.
> 
> The relevant QEMU KVM PMU paths are currently restricted to
> AuthenticAMD guests: host/guest PMU compatibility checks, AMD PMU
> information initialization, and AMD PMU MSR save/restore.  For a Hygon
> Dhyana guest with pmu=on, the compatibility check reports vPMU as
> unsupported solely because the guest vendor is HygonGenuine rather than
> AuthenticAMD.  If the VM continues, QEMU still skips AMD PMU setup and
> never saves or restores the AMD PMU MSRs for the Hygon guest.
> 
> Treat Hygon as using the AMD PMU CPUID/MSR layout for these KVM PMU
> paths.  Because the PMU MSR layout is shared, accept both AMD and Hygon
> hosts for guests using that layout.  This intentionally allows the PMU
> compatibility check to pass for AMD-host/Hygon-guest and
> Hygon-host/AMD-guest combinations.
> 
> This does not enable PMU, perfctr-core, or perfmon-v2 by default for the
> Dhyana CPU model.  On backward migration to older QEMU, Hygon PMU MSR
> state may still be dropped because older QEMU did not write that state
> back through the AMD PMU MSR KVM paths for Hygon guests.
> 
> Signed-off-by: Tina Zhang <[email protected]>
> ---
>  target/i386/kvm/kvm.c | 73 +++++++++++++++++++++++++++++++++----------
>  1 file changed, 56 insertions(+), 17 deletions(-)

The code looks good to me! It would be even better if this could be
split into two patches - one dedicated to refactoring the vendor checks
first, and another to add Hygon as a new AMD-compatible vendor.

This would make it easier for maintainers to review, and the current
commit message would more directly correspond to the Hygon-specific
changes. :)

Thanks,
Zhao